Digital printing allows rapid printing of text and graphics. The major benefit of digital printing is that it allows the rapid conversion of computer files (soft) into (hard) printed product. Unlike conventional printing using plates, digital printing is particularly economical for printing individual articles and for short print runs.
Digital printing is usually achieved by ‘rastering’, i.e. scanning a print-head back and forth, over a print medium.
One digital printing technique is known as inkjet printing Inkjet printing uses a print head equipped with at least one orifice or nozzle through which droplets of a liquid ink are ejected. Typically, an inkjet head is equipped with an array of orifices for selectively ejecting a plurality of droplets. The printer is configured to eject droplets of ink through the orifices in response to signals from a microcomputer or other controller. Digital printing is used for both monochrome and full colored printing, and also for niche applications such as printing conductive inks for electronic applications, and for printing polymer based inks for creating texture.
Monochrome printing uses one ink color, typically black. Full color printing uses at least the three primary ink colors: magenta (M), cyan (C), and yellow (Y); and usually uses four ink colors, further including black (K). Occasionally, printing may use a specialized color scheme, such as a set of inks with light colors e.g. CMYK LcLmLyLk or, CMYK plus one or more additional spot or specialized color, or any other combinations of inks, spot or specialized inks, such as white ink or gold ink for example. A full color inkjet head thus typically includes at least three separate arrays of orifices; each array of orifices being coupled to a reservoir of ink of a different color. The print head further includes a means for jetting the ink, i.e. for forcing it through the orifices and onto the medium to be printed. Jetting techniques are discussed in more detail below.
In addition to printing colors onto a substrate or medium such as paper, because of the versatility and resolutions obtainable, inkjet printers are used for a wide range of applications, and may print onto a wide range of media. Thus inkjet printers are used for
printing patterns onto ceramic tiles for application to walls and floors, such as faux-marble, and printing faux-wood in artificial veneers and parquet floor panels.
In addition to printing regular black and colored inks, inkjet printing may be used for specialized printing purposes, such as printing photo-resists, metallic conducting lines and electronic components onto printed circuit boards (PCBs) and other electronic substrates.
Polymers may be printed onto card to give a raised texture. Indeed, photopolymer jetting may be used for rapid prototyping.
To print onto a print medium such as paper, plastic, wood, aluminum, glass, ceramic tiles, fabric and the like, the inks are ejected or “jetted” from the print head, typically whilst the print head is rastered back and forth over the print medium to print across the medium. Some inkjet printers are ‘two-way’ printers, and eject ink whilst rastering in both the left-to-right direction and in the right-to-left direction. Other systems print in only one direction and are known as One-way’ printers.
Printing consists of jetting a side-to-side raster line comprising a series of dots onto the printing medium in what is sometimes referred to as the “fast scan” direction. Printing is achieved along the medium in what is sometimes known as the “slow scan” direction by either advancing the medium past the print head track, or the print head track is moved along the medium between passes.
The printed image produced by inkjet printing techniques typically consists of closely aligned rows of dots. The human eye is able to see a printed dot that is about 25 micrometers in diameter. However, an array of such dots forms a continuous image since the eye is unable to resolve the individual dots. Similarly, closely spaced dots of different colors are indistinguishable individually, and, in this manner, ink jetted printing of the three or four primary printing colors (with or without black) may provide a wide range of shades (hues) and blended colors, each of which is determined by the respective proportions of the primary colors of which it is composed.
In order to achieve a high quality printed image, not just the proportions but also the absolute amount of each color deposited in a given area is variable over a number of different intensity levels. The different intensity levels are sometimes known as ‘grey scale’, ‘dynamic range’ or ‘dynamic depth’.
In general, the drive controllers for the precise movement and location of the print heads in the fast scan direction (where applicable) and in the slow scan direction are based on programmable hardware.
In droplet on demand (DOD) inkjet printing, the inkjet print head consists of ejection chambers of ink that are coupled via feeder conduits to reservoirs of ink that may be integral to the print head and may raster with the ink head, or may be off axis and stationary. The principal types of DOD printers are valve-jet, piezoelectric, thermal (or bubble jet), and hot-melt ink printers. Two technologies described herein are piezoelectric and thermal.
In piezoelectric DOD inkjet printing, the ejection chambers are equipped with piezoelectric transducers and orifices. Signals from a controller cause the piezoelectric transducers to change their shape, applying pressure on the ink in the ejection chamber and forcing a column of ink through the orifices. The column of ink breaks into droplets.
In thermal DOD inkjet printing, a small volume of ink in the ejection chamber is superheated to vaporization temperature. A bubble of vapor forms within the ejection chamber. The bubble stimulates a fluid-pressure impulse. The impulse forces an ink droplet out of the chamber through the orifices. The upper limit for the firing frequency is determined by the overheating and fluid refill characteristics of the chamber. Firing chamber densities may currently reach a maximum resolution of 600 dots per inch (dpi) with a single pass of the print head. For most applications, 600 dpi provides acceptable text and graphics quality. Higher-quality printing is achieved with several passes of the print head, but at a cost of reduced printing speed.
In contrast to thermal inkjet, the piezoelectric inkjet print head is permanent.
Typical drop ejection frequencies are 36 kHz and can reach 100 kHz. Typical drop volumes range from 2 pL to 50 pL. Typical velocities are 10 to 15 m/s. And a typical head-to-media spacing is 1-3 mm.
The demand for high print quality is constantly growing. To enable high output, it may be useful to be able to print at high resolution with a large dynamic range (grayscale) using wide format or ultra-wide format printers.
The simplest digital printing method, known as binary printing, has two possible grey levels at each pixel point: drop or no drop; signified by 1 and 0, respectively.
In inkjet printing, the ink may be ejected through the orifice by applying a potential difference, using a pulse. The prior art includes methods that attempt to enhance the speed, grey scale and resolution capabilities of inkjet printing.
U.S. Pat. No. 7,673,965 to Mills et al. titled “Apparatus and methods for full-width wide format inkjet printing” describes using a plurality of inkjet printing heads disposed in a print head array for printing an image on a substrate at native resolution across the entire width of the substrate without scanning across the width of the substrate (abolishing fast scan). Hence the scanning time is reduced but a large number of print heads is required. However, the plurality of print heads leads to an increase in cost, a reduction in reliability and a maintenance burden.
U.S. Pat. Nos. 7,591,534, 7,588,316, and 7,585,050 to Silverbrook all refer to an increased number of nozzles with a large number of CMOS controlled print heads that supposedly improves the resolution of the resulting printing. These patents as well as several others by Silverbrook are based on a unique design of CMOS IC print heads which differs from the commonly used and popular piezoelectric print head design. However, the design has been found to be sensitive and fragile.
In U.S. Pat. No. 6,193,347 to Askeland et al., multi drop and multi pass principles are described, where a dot is created by accumulated and overlapping ink drops, thus increasing resolution.
U.S. Pat. No. 6,779,861 to Mori describes an arrangement of injecting orifices, permitting the use of smaller ink drops and a higher apparent print resolution.
The notion of two or more drop sizes that can be used to create a so-called “super pixel” providing an increased number of grey levels has been mentioned in US Patent Application Number 2002/0105557 and earlier in “Printer Handbook”, M. L. Chambers, IDG books, 2nd edition, 2000, chapter 3.
U.S. Pat. No. 7,152,946 to Desie, proposes a two-drop-size principle that may be used to enable grey level enhancement. Each print head has orifices of fixed and varying sizes, reflecting the sizes of the resulting drops. In a particular configuration, each print head has orifices of two different sizes. The method also includes controlling the drop size by drop firing at two frequencies. The grey scale is further enhanced by dithering. Desie's approach requires setting the diameter of print head/nozzles in advance.